feat: enhance dashboard with PRs, adherence, activity, progression chart, and muscle heatmap

Add 3 new stat cards (Last Workout, Personal Records, Adherence Rate),
recent activity table, progression timeline chart, and muscle group
recency heatmap to the dashboard. Remove Total Volume card.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/templates/partials/muscle_heatmap.html

@@ -0,0 +1,24 @@
+<article>
+    <header><h3>Muscle Group Heatmap</h3></header>
+    {% if muscle_recency %}
+    <div class="muscle-heatmap">
+        {% for mg in muscle_recency %}
+        <div class="muscle-heatmap-cell {% if mg.days_ago is none %}recency-never{% elif mg.days_ago <= 3 %}recency-fresh{% elif mg.days_ago <= 7 %}recency-ok{% elif mg.days_ago <= 14 %}recency-stale{% else %}recency-overdue{% endif %}">
+            <strong>{{ mg.muscle_group }}</strong>
+            <br>
+            {% if mg.days_ago is none %}
+            <small>Never worked</small>
+            {% elif mg.days_ago == 0 %}
+            <small>Today</small>
+            {% elif mg.days_ago == 1 %}
+            <small>1 day ago</small>
+            {% else %}
+            <small>{{ mg.days_ago }} days ago</small>
+            {% endif %}
+        </div>
+        {% endfor %}
+    </div>
+    {% else %}
+    <p>No exercises found in the library.</p>
+    {% endif %}
+</article>